A road map or, alternatively, a roadgraph, is a graph network of information including roads, lanes, intersections, and the connections between these features. A roadgraph also typically includes a set of prescribed driving rules (speed limits, lane or road width defaults, whether turn right on red is allowed or disallowed, driving on the left or right hand sides of the roads, etc.) The roadgraph may also include one or more zones defined as areas, typically defined as a bounded polygonal area, that embody alternative driving conventions within that zone that differ from the rules defined for the roadgraph within which the zone is defined. One example is an automated tactical, or outdoor, forklift truck where, from work area to work area, the truck operates driving missions over roadways and follows normal driving rules, but at points of load pick-up or put-down, may enter a zone where alternative driving rules can be applied (for instance the truck may drive from any point to any other point without regards to lanes or right/left hand passing rules as long as no person, other vehicle, or obstacles is placed into danger of collision).
Autonomous vehicles basically traverse roadgraphs by finding and taking paths through the roadgraph from the vehicle's current location to a designated endpoint. As the graph is traversed, vehicle drive logic generates waypoints typically expressed as GPS coordinates (latitudes and longitudes) or some other equivalent location coordinates (for instance in Universal Transverse Mercator coordinates express locations as meters from an origin point). Then the vehicle drives these points, with possible small path variations to avoid obstacles, to get to the designated endpoint (or sometimes this might be called a checkpoint).
Associated with the roadgraph there may be global driving rules and parameters. In the rules of the road for the DARPA Urban Challenge [DARPA 2007 Rules] derived from California driving rules of the road, these rules included:                Rules for passing cars stopped on the road        Rules for intersection precedence (stopping and yielding to cross traffic)        Rules prohibiting passing within a specified distance from intersections        Maximum Speed limits        
Associated with waypoints (or checkpoints) in the roadgraph there might be specific driving rules or behaviors that are invoked as the point is approached or as it is passed. In the rules of the road for the DARPA 2007 Rules, these included:                Stopping at a stop point—also anticipating other driver behaviors at stop points in other lanes connected in the graph to this stop point (i.e. collectively defining an intersection and its specific intersection behavior)        Changing speed based on speed limits defined on a particular segment of road in the graph (i.e. limits higher or lower that the default maximum speed limit for the roadgraph)        Changes in authority to deviate from the planned path for obstacle avoidance dependent on specified lane widths for a particular road segment within the roadgraph        Lane change rules for the road segment (i.e. expressed to human drivers as yellow solid lines of white solid lines that are there to prevent lane changes)        
Other rules could include whether turn right on red is allowed, whether U-turns are allowed, lane closures, special occupancy lanes (i.e. multiple occupancy lanes and perhaps in the future, autonomous vehicle lanes).
FIG. 1 shows how the DARPA roadgraph is defined [DARPA 2007 Route Network File (RNDF) and Mission Data File (MDF) Formats]. Most of the graph links the roadgraph to a series of road segments. These road segments are named, potentially have a unique speed limit, and define each lane along the road segment. The lanes are defined as a series of waypoints in GPS coordinates (but alternatively in another location defining form like UTM). Associated with each of these points can be driving rule properties like those already described. Furthermore some of the waypoints can also be defined a checkpoints that can be specified as destination locations.
Other map representations use similar encoding approaches. Ex: Google Maps codes points as KML [KeyHole Markup Language] which uses polylines to encode paths made up of compressed waypoint coding latitude, longitude and altitude. Then various properties are associated with these segments like speed limit, place names, etc. Ex: TIGER, Topologically Integrated Geographic Encoding and Referencing, was developed by the US Census Bureau and encodes path data as lines and polygons made up of latitude and longitude waypoints. Other properties are included as data blocks referencing locations or areas—how one encodes this data in a software application is left to the developer.
Note that one of the features of the DARPA graph is the definition of a zone. The idea of the DARPA zone is an area where the automated vehicle is free to drive from any zone entry point to any checkpoint defined in the zone or to a designated exit point from the zone through any path (usually the shorted path) that is free of obstacles. The DARPA zone models driving off road over an area of rugged terrain, but it are also an ideal representation for:                Parking areas        Intersections (based on intersection precedence the automated vehicle will enter the intersection expecting all other drivers to respect their own yielding protocol so that should not be any other vehicle or pedestrian in the intersection that would interfere with the autonomous vehicle, however, as we all know some people violate these precedence rules creating the possible situation in the intersection where vehicles have to maneuver around each other and pedestrians or stop so as not to collide with them—this is the basic zone driving behavior, take any path out of the zone that does not collide with another vehicle, pedestrian, or obstacle).        Construction zones (using barriers that are set-up as obstacles to guide traffic around work areas)        
In U.S. Pat. No. 8,509,982, the entire content of which is incorporated herein by reference, the concept of a zone is defined as locations where diving is challenging such as merges, construction zones, or other obstacles where the automated driving system is not capable. This example is that in a zone, driving rules might require the autonomous vehicle to alert the driver that the vehicle is approaching the zone so that the driver can take control of steering, acceleration, etc. In another example they suggest that entering a zone might be associated with an automatic broadcast from the vehicle to others in the surrounding area signaling an unsafe condition.